1. Field of the Invention
The invention relates to an apparatus and a method for use in instrument landing system (ILS) localizer and/or glide-slope receivers for deriving course deviation information.
2. Description of the Prior Art
Pilots often fly aircraft under conditions, such as adverse weather, which necessitate flying under instrument flight rules (so-called IFR conditions). An essential component of such flight is reliance on an instrument landing system (ILS). This system, which includes a number of ground-based transmitters and various ILS receivers installed in the aircraft, provides the pilot with appropriate navigation information to enable the pilot to place his aircraft on a proper inbound (or in certain instances outbound) course to (or from) a runway. In particular, an ILS system typically provides the pilot with information which indicates the aircraft's vertical and horizontal deviation from one or more pre-selected approach paths to (or from) a particular runway at a desired airport. The "glide-slope" ILS receiver derives the vertical deviation information; while the "localizer" ILS receiver derives the horizontal deviation information. These deviations are, in turn, displayed on appropriate indicators, usually meters or the like, and may also be fed to appropriate on-board auto-pilot systems.
An Instrument Landing System typically consists of three ground-based (localizer, glide-slope and marker beacon) transmitters and suitable ILS receivers mounted in the aircraft. The ground-based localizer transmitter located at a far end of a runway radiates, on a selected one of 40 channels spaced 50 kHz apart between 108-112 mHz, two antenna patterns which together provide an equisignal course along and extending out beyond the centerline of the runway. In particular, the left-hand pattern is amplitude modulated, typically by 20%, by a 90 Hz tone (or signal); while the right-hand pattern is amplitude modulated, also at approximately 20%, by a 150 Hz tone (or signal). The airborne ILS localizer receiver detects both of these tones (hereinafter referred to as the localizer pair), rectifies the results and presents a left/right deviation display on a zero-center DC meter situated in the cockpit of the aircraft. The ground-based glide-slope transmitter located at an approach end of a runway radiates, on a pre-determined channel (paired with the localizer channel) in the 330-335 mHz band, two antenna patterns which provide an equisignal course oriented along an approximate 3 degree incline emanating from the approach end of the runway. The lower pattern is amplitude modulated, typically also by 20%, by a 150 Hz tone; while the upper pattern is amplitude modulated, also at approximately 20%, by a 90 Hz tone. The airborne ILS glide-slope receiver, in much the same manner as the ILS localizer receiver, detects both of these tones (hereinafter referred to as the glide-slope pair), rectifies the results and presents an above/below deviation display typically on another zero-center DC meter. Both the localizer and glide-slope meter needles are visably mounted within the same housing and are perpendicuarly oriented with respect to each other. Such a combined meter is commonly referred to as a "cross-pointer" display.
Whenever an aircraft is "on course" (both horizontally and vertically), i.e. following an appropriate ILS approach, to an ILS equipped runway, both the ILS localizer and glide-slope receivers will detect equal levels of the 90 and 150 Hz modulating tones. Hence, for this instance, no deviation exists and none will be displayed. However, if the aircraft is situated on an inbound course which deviates to the right of the localizer course (runway centerline), the amplitude of the 150 Hz tone in the localizer pair will be greater than that of the 90 Hz tone in that pair. Alternatively, should the aircraft be situated on an inbound course which deviates to the left of the localizer course, then the amplitude of the 90 Hz tone in the localizer pair will predominate over that of the 150 Hz tone in that pair. Furthermore, if the aircraft is situated above a selected glide-path, then the 90 Hz modulating tone in the glide-slope pair will possess a greater amplitude than the 150 Hz modulating tone in that pair. Likewise, if the aircraft is below the glide-path, then the amplitude of the 150 Hz tone in the glide-slope pair will predominate instead. In each instance, appropriate visual indications of any actual vertical and horizontal deviations, from the selected ILS approach, will be derived by the appropriate ILS receiver and displayed on the cross-pointer display.
Generally, the signal analyzing circuitry of both the ILS localizer and glide-slope receivers known to the art is rather simple and is the same for both receivers. This circuitry typically comprises a suitable rf (radio frequency) front end, an AM (amplitude modulated) detector, 90 and 150 Hz band-pass filters, rectifiers, a DC (direct current) comparator and one of the indicators of a cross-pointer display. In particular, for either receiver, the appropriate incoming ILS modulated carrier signal is first amplified, de-modulated and then filtered to yield the 90 and 150 Hz navigation tones. Each resulting navigation tone is then rectified to produce a DC voltage proportional to the peak amplitude of the tone. Thereafter, the comparator determines the difference between the amplitudes of the two DC voltages associated with both navigation tones and, in turn, applies a voltage indicative of course deviation to the appropriate indicator in the cross-pointer display.
Although these analyzing circuits are quite simple, they only provide accurate results in the absence of interfering signals. Unfortunately, interference can not be avoided. In fact, the performance of existing ILS glide-slope and localizer receivers, typified by that described above, continually degrades in the presence of increasing interference and is thus often only marginal at best. In particular, whenever an interfering signal is present along with the navigation tones in either of these ILS receivers, this interfering signal is attenuated somewhat by both the 90 and 150 Hz band-pass filters. The remaining portion of the interfering signal present at the output of these filters adds to the amplitude of the 90 and 150 Hz navigation tones. In the remote event that this portion presents the same amplitude at the output of both the 90 and 150 Hz filters, then this interference would not adversely affect the course deviation information. However, most interfering signals will be unequally attenuated by these filters and will thus affect the amplitude of one of the navigation tones, i.e. either the 90 or the 150 Hz tone, to a greater degree than the other. For example, an interfering signal might contain a 210 Hz audio component. In this case, the 150 Hz band-pass filter might attenuate this 210 Hz signal to a level of approximately 125 mV (millivolts); while the 90 Hz band-pass filter would generally attenuate this audio signal to a lower level, such as 40 mV. As a result, an 80 mV imbalance would exist. Since, most course deviation meters only require 150 mV for full scale deflection, this imbalance would drive the meter to an erroneous half scale deflection. As the frequency of the interfering signal approaches 150 Hz, the imbalance becomes greater. Likewise, similar interference and erroneous deviation indications would be produced from interfering signals that contain audio components below 90 Hz and even between 90 and 150 Hz.
Unfortunately, presently available ILS receivers, particularly the localizer receiver, are particularly susceptible to interference from multiple signals, such as those generated from commercial FM broadcast stations. This interference can result from spurious signals generated from either a commercial FM broadcast station itself or intermodulation frequencies generated by the interaction of multiple broadcast frequencies transmitted from a common antenna, or even from the interaction of the transmissions from two or more separate FM broadcast signals that are both situated close to the 108-112 mHz band used for localizer transmissions wherein at least one of the FM stations is sufficiently powerful to overload the ILS localizer receiver and drive it into non-linear operation. Fortunately, as the result of the efforts of the Federal Communications Commission in not authorizing operation of a powerful FM station on a frequency in close proximity to that of an existant ILS localizer transmitter, no serious FM interference exists in ILS systems operating in the United States. However, as the result of a worldwide re-allocation of radio spectrum which occurred in 1979 and which extended the upper end of the world-wide FM broadcast band from 100 mHz to 108 mHz, powerful new FM broadcast stations which will operate on frequencies close to 108 mHz are expected to be built in Europe, Africa, and parts of the Middle East. Hence, these new stations are expected to impose serious FM interference to the ILS systems operating in these areas and thus jeopardize the safety of aircraft operating there. See, e.g., Klass, "New Stations May Disrupt Air Navigation", Aviation Week and Space Technology, Sept. 5, 1983; pages 33-34.